Dualsend SMTP Failover

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How does failover work?

When the primary server does not accept a message, the plugin sends it through the fallback server straight away, with no waiting in the page. If the fallback server fails too, the message is queued for a background retry (see below). Fallback settings saved by version 1.0.0 are used as they are.

How do background retries work?

When both servers fail, the email is stored in a queue and the page that sent it carries on at once. A WP-Cron task tries the email again every Retry Interval minutes (1 to 60, 5 by default), through the primary server and then the fallback server, up to Maximum Retries times (0 to 10, 3 by default). After the last retry fails, the email is handed to WordPress mail as a last resort. Set Maximum Retries to 0 to turn retries off; the email then goes to WordPress mail at once, as it did before. The Delivery and Diagnostics card shows how many emails are waiting and when the next try is. Retry values saved by version 1.0.0 are used as they are.

At most 100 emails wait at once. When the queue is full, or the attachments of an email are larger than 10 MB in total, that email goes to WordPress mail at once instead.

WP-Cron runs when your site gets visits. If your site sets DISABLE_WP_CRON, make sure a real server cron job runs wp-cron.php, or queued email waits until it does.

What does the retry queue store, and for how long?

While an email waits for a retry, the whole email is stored in your WordPress database: the recipients, subject, headers and body. A body can hold private content, such as a password reset link. Attached files are copied into a private folder inside your uploads folder, under random names, with an empty index.php and a rule that blocks web access on Apache servers. On servers that do not read .htaccess files, such as nginx, the random names keep the files from being found; you can also add a deny rule for the dualsend-smtp-failover-queue folder.

The email and its copied files are deleted as soon as the email is delivered or handed to WordPress mail after the last retry. Copies left behind by a request that stopped before it could queue its email are deleted by the next background run, one hour after they were made. Nothing in the queue is sent anywhere except to your own mail servers and, as a last resort, WordPress mail. Nothing is ever sent to the plugin author. Deleting the plugin removes the queue and the folder. Set Maximum Retries to 0 to keep nothing.

My host blocks SMTP ports. What do I do?

Choose a provider that offers an API, such as SendGrid, Brevo, Postmark, Mailgun and most others in the list above, and set the connection type to API. API delivery runs over ordinary HTTPS, which is not blocked.

Where are my passwords stored?

In the WordPress options table, in the same database as the rest of your site, and only readable by users who can already manage plugin settings. They are never sent back to the browser once saved, and they are masked before anything is written to the debug log.

For extra safety on a shared host, keep your provider’s API key scoped to sending only, rather than full account access.

Why did my message send but land in spam?

Almost always because the From address does not match a domain your provider is authorized to send for. Set up SPF, DKIM and DMARC records with your provider, and use a From address on that domain.

Does it work with Contact Form 7?

Yes, and it treats form submissions specially: while a visitor is waiting on a form, timeouts are shortened, so a slow mail server never leaves the page hanging.

Any plugin that sends through wp_mail() works, including WooCommerce, Gravity Forms, WPForms and the WordPress core notifications.

Will it interfere with other plugins that set their own sender address?

Not unless you ask it to. By default a plugin that sets its own From address keeps it. Switch on Override other plugins on the Settings tab if you want your configured address used everywhere. Until a delivery route is set up, the plugin does not change the sender of any email.

Can I keep another SMTP plugin active, such as WP Mail SMTP, FluentSMTP or Post SMTP?

It works, but only one of them sends your email. While Dualsend SMTP Failover has a delivery route, it takes over wp_mail() before the other plugin, so the mail settings of the other plugin are not used. Until it has a route, the other plugin sends as before. The settings page shows a short notice, which you can dismiss, while another SMTP plugin is active. Keeping one SMTP plugin active avoids confusion.

Does it send inline images?

Yes. Since WordPress 6.9, wp_mail() can embed images that the message refers to with cid:. Such a message is sent over SMTP, on the primary server and then the fallback server, because the provider APIs are not used for embedded images. When both servers fail, it goes to WordPress mail at once instead of the retry queue.

What happens when both servers fail?

With background retries on, the message is queued and tried again later, as described above. After the last retry, or at once when Maximum Retries is 0, it is handed to WordPress, which tries its own PHP mail handling. Switch on the debug log to see exactly what failed and why. The log never records message bodies.